Butanol is an important industrial chemical, useful as a fuel additive, as a feedstock chemical in the plastics industry, and as a foodgrade extractant in the food and flavor industry. Each year 10 to 12 billion pounds of butanol are produced by petrochemical means and the need for this commodity chemical will likely increase.
Isobutanol is produced biologically as a by-product of yeast fermentation. It is a component of “fusel oil” that forms as a result of incomplete metabolism of amino acids by this group of fungi. Isobutanol is specifically produced from catabolism of L-valine in the cytoplasm. After the amine group of L-valine is harvested as a nitrogen source, the resulting α-keto acid is decarboxylated and reduced to isobutanol by enzymes of the so-called Ehrlich pathway (Dickinson et al., J. Biol. Chem. 273(40):25752-25756 (1998)). Yields of fusel oil and/or its components achieved during beverage fermentation are typically low. For example, the concentration of isobutanol produced in beer fermentation is reported to be less than 16 parts per million (Garcia et al., Process Biochemistry 29:303-309 (1994)). Addition of exogenous L-valine to the fermentation increases the yield of isobutanol, as described by Dickinson et al., supra, wherein it is reported that a yield of isobutanol of 3 g/L is obtained by providing L-valine at a concentration of 20 g/L in the fermentation. However, the use of valine as a feed-stock would be cost prohibitive for industrial scale isobutanol production.
Disclosed in US Patent Publication US20070092957 A1 is engineering of microorganisms for expression of several isobutanol biosynthetic pathways. Additionally Van Nedervelde et al (Proceedings of the Congress—European Brewery Convention (2003), 29th, 50/1-50/10) have demonstrated that deletions of the gene encoding the BAT1 mitochondrial protein in yeast resulted in strains having increased levels of higher alcohols. Similarly Nako et al (WO 2007032522) note that amyl alcohol and/or isobutanol and/or isoamyl acetate levels in yeast used for the production of alcoholic beverages may be altered via manipulation of the BAT1 and BAT2 genes. The art is silent with respect to the down regulation of other genes encoding proteins that are functional in the mitochondria for the enhanced production of isobutanol in yeast.
There is a need for attaining higher amounts of isobutanol through yeast fermentation without addition of valine or other isobutanol production intermediates.